Transmitter and receiver units for spatial position measurement system

ABSTRACT

Improved transmitter and receiver units for use in spatial measurement system that are easy and inexpensive to manufacture while providing a high degree of reliability are disclosed. Specifically, the laser transmitter includes a laser emitter, a bearing/motor assembly coupled to the laser emitter, the bearing/motor assembly including a rotatable hollow spindle shaft through which a laser beam generated by the laser emitter passes and a motor for driving the spindle shaft, a prism assembly coupled to the spindle shaft, wherein the prism assembly divides the laser beam generated by the laser emitter into a pair of fanned laser beams, and reflecting means for reflecting the fanned laser beams generated by the prism assembly as counter-rotating fanned laser beams. The receiver unit preferably includes at least one optical receiver coupled to an extension member, a processing unit coupled to the optical receiver, and a receiver interface coupled to the processing unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation of application Ser. No. 08/123,891 filed Oct. 5,1995 which in turn is a continuation-in-part of U.S. patent applicationSer. No. 07/714,891, filed Jun. 17, 1991, entitled "Spatial MeasurementRecovery System" by Beliveau et al., Notice of Allowance received Mar.23, 1993, and a continuation-in-part of U.S. patent application Ser. No.07/755,780, filed Sep. 6, 1991, entitled "Spatial Positioning System" byDornbusch et al., both of which are incorporated herein by reference,now U.S. Pat. Nos. 5,461,473, 5,247,487 and 5,294,970, respectively.

FIELD OF INVENTION

The present invention relates to a spatial position measurement systemand, more particularly, to a spatial position measurement system whichprovides three-dimensional position measurement information of an objectusing at fixed transmitter units and at least one portable positionsensor unit.

BACKGROUND OF INVENTION

Computer Aided Design and Drafting (CADD) has become a standard designtool for manipulating and recording design information in manyindustries including construction and manufacturing. CADD models areused to accurately depict the position, shape and orientation ofobjects, and their relationship to other objects, within a definedenvironment. The ability to model the objects with the definedenvironment greatly reduces the amount of engineering effort that mustbe expended in devising a working design that can be efficiently andeconomically implemented.

U.S. Pat. Nos. 5,100,229 and 5,110,202, the contents of both of whichare incorporated herein by reference, for example, describe systems andmethods in which a CADD model is used in connection with a portableposition sensor and at least one referent station to identify specificpoints within an environment. The position information can be used tocontrol construction equipment or model different variations of proposeddesigns based on the actual measured environment. U.S. Patentapplication Ser. No. 07/714,891 further describes a system for gatheringspatial and descriptive data on existing structural elements within anenvironment that can be used to build CADD models. In order to becommercially viable, however, the position measurement systems describedabove must be manufactured in an efficient an economical manner thatpermits their implementation on a wide scale with a high degree ofreliability.

It is an object of the invention to provide improvements in thetransmitter units and receiver units of the basic systems describedabove, in order to make the systems easier and less expensive tomanufacture while maintaining a high degree of reliability.

SUMMARY OF INVENTION

The invention provides improved transmitter and receiver units for usein spatial measurement system that are easy and inexpensive tomanufacture while providing a high degree of reliability. Specifically,a laser transmitter unit for a spatial measurement system is disclosedthat includes a laser emitter, a bearing/motor assembly coupled to thelaser emitter, the bearing/motor assembly including a rotatable hollowspindle shaft through which a laser beam generated by the laser emitterpasses and a motor for driving the spindle shaft, a prism assemblycoupled to the spindle shaft, wherein the prism assembly divides thelaser beam generated by the laser emitter into a pair of fanned laserbeams, and reflecting means for reflecting the fanned laser beamsgenerated by the prism assembly as counter-rotating fanned laser beams.The receiver unit preferably includes at least one optical receivercoupled to an extension member, a processing unit coupled to the opticalreceiver, and a receiver interface coupled to the processing unit.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be described in greater detail with reference to theaccompanying drawings, wherein:

FIG. 1 is a drawing illustrating the use of at least two a lasertransmitter units to scan an environment to be modeled;

FIG. 2 is a drawing of a first embodiment of a portable receiver unit;

FIG. 3 is a drawing of a second embodiment of a portable receiver unit;

FIG. 4 is a top cut-a-way view of the laser transmitter unit illustratedin FIG. 1;

FIG. 5 is a side cut-a-way view of the laser transmitter unitillustrated in FIG. 1;

FIG. 6 is a partial cut-a-way view of the scanner module illustrated inFIGS. 4 and 5;

FIG. 7 is an illustration of prism cubes utilized in the prism assemblyshown in FIG. 6;

FIG. 8 is an illustration of the prism cubes shown in FIG. 7 withcylindrical lens and dummy lens bonded thereto;

FIG. 9 is a schematic diagram of a first embodiment of an opticalreceiver;

FIG. 10 is a schematic diagram of a second embodiment of an opticalreceiver;

FIG. 11 is a top view of the circular array used in the optical receiverillustrated in FIG. 10;

FIG. 12 is a schematic view of a parabolic reflector and spherical lensthat can be used to replace the reflective cone in the embodimentsillustrated in FIGS. 9 and 10;

FIG. 13 represents a cross-sectional view of a complete opticalreceiver.

FIG. 14 is a diagram of a telescopic receiver unit; and

FIG. 15 illustrates the common location of two transmitter units on arotating device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A spatial position measurement system, in accordance with the invention,preferably includes a minimum of two laser transmitter units 100 mountedon tripods and arranged at points in an environment to be modeled asshown in FIG. 1. At least one portable receiver unit 200, of a typeshown in FIGS. 2 or 3, is used to determine the location of particularpoints or structures within the environment. Each laser transmitter unit100, which constitutes a fixed referent station, producescounter-rotating fanned laser beams, which sweep over an angle (A) shownin a top view of a laser transmitter unit 100 illustrated in FIG. 4, ata constant angular velocity. The portable receiver unit 200 generatesposition data based on the detection of the fanned laser beams emittedfrom the laser transmitter units 100. The position data generated by theportable receiver unit 200 is used to model an environment as describedin U.S. patent application Ser. No. 07/714,891, the contents of whichhas been incorporated by reference.

The laser transmitter unit 100, as illustrated in FIGS. 4 and 5,preferably includes a scanner module 102 provided within a transmitterhousing 104 having a preferably cylindrical exit window 106 attachedthereto. The center of the cylindrical exit window 106 is aligned withthe virtual rotation axis of a spinning prism assembly 108 of thescanner module 102. The exit window 106 is potted into a bezel which isthen mounted to the transmitter housing 104 with an o-ring seal. The useof the cylindrical window 106 reduces the size of the housing 104required for a given sweep angle, and also allows the emitted fannedlaser beams to pass through with a minimum amount of beam distortion.The transmitter housing 104 is preferably hermetically sealed utilizinga positive pressure of dry gas (preferably nitrogen), which allows thelaser transmitter unit 100 to operate satisfactorily under virtually allweather conditions without fogging or other degradation of performance.Hermetic connectors 110 are used for power and communicationsinterfaces.

The scanner module 102, as shown in greater detail in FIG. 6, includes alaser emitter 112 coupled to a bearing/motor assembly 114 that drivesthe scanning rotating prism assembly 108, such that the spin axis of thebearing/motor assembly 114 and the optical axis of the laser emitter 112are coaxially arranged. The bearing/motor assembly 114 includes a drivemotor 116 coupled to a hollow spindle shaft 118 which is supported by ahydrodynamic gas bearing 120. A collimated laser beam, generated by thelaser emitter 112, passes through the center of the spindle shaft 118and into the spinning prism assembly 108, which generates a pair offanned laser beams. A 90 degree fold mirror 122, preferably a singlepiece that is kinematically mounted onto the end of the bearing/motorassembly 114 to surround the spinning prism assembly 108, diverts thefanned laser beams out through the cylindrical exit window 106 shown inFIG. 4. Two pairs of counter-rotating fanned output beams are generatedby the transmitter unit 100, as the two fanned beams emitted by theprism assembly 108 sweep across a top surface of the fold mirror 122 inone direction to generate the first pair of output beams, and then sweepacross a bottom reflective surface of the fold mirror 122 in an oppositedirection to generate the second pair of output beams.

The hydrostatic gas bearing 120 provides a near frictionless bearingsurface and essentially eliminates cogging torque. In addition, thesmall clearances and large contact areas inherent in the hydrodynamicbearing 120 design limit susceptibility to damage from handling shockwhen the unit is not operating, as opposed to traditional ball or rollerbearings which suffer from "brinneling" of bearing surfaces whensubjected to shock. Brinneling degrades the bearing surface over timeand can significantly limit rotational stability.

The scanning prism assembly 108 preferably includes two prism cubes 124,126, each split at 45 degrees as shown in FIG. 7, that are bondedtogether one on top of the other. The first prism cube 124 includes abeamsplitter 90 degree fold. The reflected portion of the collimatedlaser beam received from the spindle shaft 118 exits through acylindrical lens 128, bonded to a first side of the prism cube 124,which spreads the beam into a fan in one plane to generated a firstfanned beam. A dummy cylindrical lens 130 is bonded to a second side ofthe prism cube 124, opposite to the side having the cylindrical lens128, to restore balance as shown in FIG. 8. The transmitted portion ofthe beam then enters the second prism cube 126, which is opticallycemented on top of the first prism cube 124, is reflected through acylindrical lens 132 bonded on a first side of the second prism cube 126to generate a second fanned beam. A dummy cylindrical lens 134 is alsoprovided on the second prism cube 126 to restore balance. The secondprism cube 126 is rotated through an angle phi (preferably, 35 degrees)relative to the first prism cube 124 in the plane in which they arebonded to offset the first fanned beam from the second fanned beam. Thefirst cylindrical lens 128 is rotated an angle gamma (preferably 45degrees) relative to the axis of rotation in order to slant the secondfanned beam with respect to the first fanned beam. The completedscanning prism assembly 108 is cemented and mechanically fixed to astainless steel mount 123 which is rigidly anchored directly to thehollow spindle shaft 118.

Experimentation has demonstrated that a system using the transmitterunits 100 to determine position can function properly using only threeemitted fanned laser beams instead of four. Thus, a portion of the 90degree fold mirror 122 can be made non-reflective, or a different shapecan be used, to eliminate one of the beams, preferably a slanted beam,from the output of the transmitter unit 100. The elimination of theslanted beam reduces the amount of timing data which must be processedby the portable receiving unit 200 to determine position.

The modular approach to the transmitter system 100 design effectivelyremoves the transmitter housing 104 from the optical alignment of thesystem. The scanning module 102 can therefore be independently assembledand aligned prior to installation within the transmitter housing 104,where it is kinematically mounted to a cradle.

The portable receiver unit 200 is preferably configured in a firstembodiment, as shown in FIG. 2, to include two optical receivers 500, aprocessing unit 204, an extension member or rod 203, a tool 205removably coupled to the rod 203, a communications link 202, and areceiver interface 201. The general interaction and operation of theillustrated components is described in detail in the above referencedU.S. patent application Ser. No. 07/714,891. The illustrated embodiment,however, utilizes the attachment of the processing unit 204 between thetwo optical receivers 500 to unify the majority of the mass of thereceiver system into a single rigid unit.

Alternatively, the portable receiver unit 200 may employ only a singleoptical receiver 500. Again, the spatial position of the opticalreceiver 500 is determined as described in prior referencedapplications. However, in the implementation illustrated in FIG. 3, theposition of the tool 205 is calculated by the processing unit 204 usingan mathematical algorithm which projects a vertical line from theoptical receiver 500 through the tool 205. In order to determine theposition of the tool 205 correctly, the optical receiver 500 must bepositioned vertically above, or below, the tool 205. The properpositioning of the optical receiver 500 with respect to the tool 205 isaccomplished using a level 205, which indicates to the operator ofportable receiver unit 200 when the optical receiver 500 is positionedvertically with respect to the tool 205. This implementation has theadvantage of a reducing the processing required to calculate theposition of the tool 205. Hence, a lighter, less expensive, less powerconsumptive processing unit 204 detached from the rod 203 may beemployed.

The purpose of the optical receiver 500 is to detect the fan laser beamsemitted by the laser transmitter units 100. The position measurementsystem described in the above patents and applications incorporated byreference employ an optical receiver that can receive light over 360degrees in azimuth and a more limited angle in elevation. A simpleembodiment of a preferred optical receiver 500, shown in FIG. 9, uses areflective cone 510, a compound parabolic collector (CPC) 501 and asingle photodiode detector 503. The reflective cone 510 directs incominglight 504 into the CPC 501. The CPC 501 in turn directs the incominglight of a limited entrance (e.g., +/-10 degrees) onto the photodiode503. This embodiment of the optical receiver 500 works well inenvironments where ambient light is at low levels and does notcontribute significantly to system noise levels.

If ambient noise is of a level which will begin to degrade rangeperformance (e.g., operation in sunlight), the structure of the opticalreceiver 500 can be modified to maintain performance. An opticalbandpass filter 505, selected to match the wavelength of the emittedfanned laser beams 150, may be placed before the CPC 501. The use of abandpass filter 505 reduces the level of ambient light entering the CPC501 and hence increases the signal-to-noise ratio over a similarnon-filtered optical receiving system 500. Additionally, multiplephotodiodes 503 may be used together in the design to overcome noisefrom ambient light. A circular array 506 of photodiodes 503 and CPCs 501may be used to reduce the field of view of any one photodiode and hencereduce the ambient noise as shown in FIGS. 10 and 11. Finally, thereflective cone 510 can also be replaced with a parabolic reflector 507located within a spherical lens 508 as illustrated in FIG. 12.

FIG. 13 illustrates a cross-sectional view of a complete opticalreceiver 500 that utilizes a circular array of compound paraboliccollectors 501, a narrow band width optical filter 505, and array of PINphotodiodes 503, and a reflective cone 510. Reflective plates 511extending from the reflective cone 510 to the optical filter 505 arealso preferably used to reflect light of the received laser beams intothe individual CPCs 501 of the circular array. Hence, the reflectiveplates 511 increase the optical receivers 500 sensitivity and increasethe usable range of the portable receiver unit 200.

The array of photodiodes 503 preferably in employs twelve photodiodes,the output of each is coupled to a low noise preamplifier provided on acircuit board 513 located beneath the array. The signal from eachpreamplifier is supplied to a threshold circuit which has a variablethreshold level. The purpose of the threshold circuit is to ensure thatonly pulses from the preamplifier corresponding to a laser beam strikinga segment of the optical receiver are sent to the processing unit 204for analysis. The threshold is set so that the peaks of electronic noisefrom the preamplifier do not cross the threshold, and thus false pulsescaused by noise are eliminated. The threshold is variable so that underconditions of unusually low noise, for example when the receiver isoperated in very low ambient light levels, the threshold can be loweredto increase the sensitivity of the receiver and therefore themeasurement range of the system. Under conditions of unusually highnoise, for example when sunlight enters directly into a receiversegment, the threshold can be raised to prevent excess noise from thatsegment from swamping the system with false pulses.

The twelve outputs from the threshold circuits are added together in asumming circuit. This effects an "OR" that any segment of the detectorwhich is hit by a laser beam sends a pulse to the output of the summingcircuit. The pulses are then taken to the input of a timing circuitwhich records the start and end time of each pulse. Under normaloperating conditions there will be typically six separate pulses in agiven time period, defined by the mean rate of rotation of the outputlaser beams from the transmitter units 100. The start and end times ofthe pulses are used to establish the center point of the pulse (i.e. thetime at which the center of the pulse was incident upon the opticaldetector) and then calculate the intervals between pulses, Thecalculations are performed by the processing unit 204.

The intervals between pulses are used to establish the actual rotationrate of the beams received from each laser transmitter 100 and therelative timing between these beams, as observed by the portablereceiver unit 200 at its particular location. The angles subtended bythe optical receiver are then calculated and used to compute theposition of the portable receiver unit 200 in the appropriate coordinatesystem.

In alternative implementations of the optical receiver as shown in FIG.9, only one photodiode is utilized. The single photodiode may receivelight pulses from a preferred direction or it may receive light from alldirections using suitable optics to direct the light to a singlereceiver. In these implementations, only one photodiode, preamplifierand threshold circuit are required and there is no summing circuit. Theoutput of the threshold circuit is taken directly to the input of atiming circuit such as described above.

The prior systems disclosed in the above incorporated patents and patentapplications utilize portable receiving units that have opticalreceivers for which receive input from 360 degrees. A portable receiverunit based on an optical telescope that accepts light from one preferreddirection, however, can be used to improve the range over which accurateposition measurement can be made. The telescope receiver requires aphotodiode in place of the conventional eye piece, or by use of a halfsilvered mirror can incorporate a photodiode which receives part of thelight entering the telescope. Additionally, the telescope may be aconventional transit or theodolite or electronic distance measuringdevice commonly use in surveying.

FIG. 14, for example, illustrates a telescope based receiver mountedthat includes a laser signal receiving telescope 600 mounted on asupport arm 602 with a horizontal pivot 603 and a vertical pivot 604.The laser signal receiving telescope 600 includes an optical system 601that focuses incoming radiation onto a photodetector 605. The output ofthe photodetector 605 is supplied to signal processing electronics 606,similar to the electronics described above, which in turn is coupled toa position calculating computer 607 that calculates the position of thelaser signal receiving telescope 600 based on the output signal suppliedby the processing electronics 606. An optical targeting telescope 608 ispreferably provided to allow the user to properly aim the telescope at atransmitter unit 100.

To perform position location measurements with a telescope basedreceiver, the telescope 600 is pivoted at one laser transmitter unit100, and the time of arrival of light from that transmitters rotatingbeams is recorded. The telescope 600 is then pointed at a second lasertransmitter unit 100 and similar recordings are made. The positioncalculating computer 607, as used in other implementations of priorsystems, converts the recorded time arrival of light from eachtransmitted beam into coordinates for the location of the telescopebased receiver.

As shown in FIG. 15, a variation on the basic systems employing theabove-described transmitter units 100 can be implemented using twotransmitter units 100 which that rigidly fixed together through amounting device 701. The mounting device 701 precisely fixes distancebetween the transmitters 100 and their relative orientation to oneanother. This implementation eliminates the need for calibration of theposition measurement system to determine the relative locations andorientations of the transmitters. Hence, the receiver system 200 mayoperate immediately to perform position measurement without the initialdelay of calibration.

The mounting device 701 is fixed to a rotation device 702 which can bemechanically controlled to turn a specified vertical or horizontalangle, or given a reorientation in the vertical or horizontal angle, thedevice must be able to measure the turned angles. The system ispreferably automatically controlled to turn a specific angle, or atleast to turn an angle in a specific direction. The system can also be amanual system whereby an individual can manually turn a horizontal angleand/or a vertical angle and an electronic system will measure and recordthe turned angle. The measured angles or changes in vertical andhorizontal can then be transmitted to a the receiver unit 200.Transmission of this information can be accomplished by manual datatransfer and any data transfer mechanism including but not limited topacket radios, and encoded information on the emitted fanned laserbeams.

The invention has been described with reference to certain preferredembodiments thereof. It will be understood, however, that modificationsand variations are possible within the scope of the appended claims.

What is claimed is:
 1. A laser transmitter unit for a spatialmeasurement system comprising: a laser emitter; a bearing/motor assemblycoupled to the laser emitter, the bearing/motor assembly including arotatable hollow spindle shaft through which a laser beam generated bythe laser emitter passes and a motor for driving the spindle shaft; aprism assembly coupled to the spindle shaft, wherein the prism assemblydivides the laser beam generated by the laser emitter into a pair offanned laser beams; and reflecting means for reflecting the fanned laserbeams generated by the prism assembly as counter-rotating fanned laserbeams.
 2. A laser transmitter unit as claimed in claim 1, furthercomprising a cylindrical exit window through which the fanned laserbeams are reflected by the reflecting means.
 3. A laser transmitter unitas claimed in claim 1, wherein the reflecting means comprises a 90degree fold mirror.
 4. A laser transmitter unit as claimed in claim 1,wherein the prism assembly includes a first beam splitting prism coupledto a second reflective prism.
 5. A laser transmitter unit as claimed inclaim 1, wherein a light emitting side of the first beam splitting prismis offset from a light emitting side of the second reflective prism,with respect to an axis of rotation of the prism assembly, by an anglephi.
 6. A laser transmitter unit as claimed in claim 5, wherein thelight emitting side of the first beam splitting prism and the lightemitting side of the reflecting prism each include a cylindrical lens.7. A laser transmitter unit as claimed in claim 6, wherein thecylindrical lens of the first beam splitting prism is rotated withrespect to the cylindrical lens of the second reflecting prism by anangle gamma.
 8. A laser transmitter unit as claimed in claim 6, furthercomprising dummy cylindrical lenses bonded to second sides of the firstbeam splitting prism and the second reflecting prism opposite to thelight emitting sides thereof.
 9. A laser transmitter unit as claimed inclaim 1, wherein the reflecting means comprises a ninety degree foldmirror.
 10. A laser transmitter unit as claimed in claim 1, wherein thebearing/motor assembly includes a hydrodynamic gas bearing.
 11. Aspatial measurement system comprising: at least two transmitter unitsand at least one receiver unit, wherein the transmitter units include alaser emitter; a bearing/motor assembly coupled to the laser emitter,the bearing/motor assembly including a rotatable hollow spindle shaftthrough which a laser beam generated by the laser emitter passes and amotor for driving the spindle shaft; a prism assembly coupled to thespindle shaft, wherein the prism assembly divides the laser beamgenerated by the laser emitter into a pair of fanned laser beams; andreflecting means for reflecting the fanned laser beams generated by theprism assembly as counter-rotating fanned laser beams; and wherein thereceiver unit includes at least one optical receiver coupled to anextension member; a processing unit coupled to the optical receiver; anda receiver interface coupled to the processing unit.
 12. A spatialmeasurement system as claimed in claim 11, wherein at least twotransmitter units are commonly mounted on a rotation device.
 13. Aspatial measurement system as claimed in claim 11, wherein the receiverunit is a telescopic receiver unit.